In the field of electrical power systems it is important to detect and locate faults safely in order to be able to perform necessary measures such as disconnect a section of the system, like to disconnect a power line. The fault handling devices that are used for this are often termed protective relays.
Such a device is normally connected to a measurement node of the electrical power system, where the measurement node may be provided in a power line. When detecting a fault it is then necessary to determine the direction of fault in relation to the node. The way this has traditionally been done is through measuring and storing the voltage before a fault occurs. Then when a fault is detected this pre-stored voltage is used as a reference voltage together with the measured current to determine the direction of the fault. This direction can then be used in locating the position of the fault in the system. Such a device is for instance described in U.S. Pat. No. 4,731,689.
One problem with this approach is that if for instance the power system is set in motion through starting to swing or changing frequency then the reference voltage is unreliable.
There is therefore a need for being able to safely determine the direction of the fault in an electrical power system also under these circumstances.